Diseases that affect the production of mature erythrocytes have a major impact on public health. One of our long term goals is to understand the signal transduction pathways that regulate terminal erythroid differentiation. Towards that end, we have adopted a viral/murine model of erythroid differentiation known as Friend disease. In the previous funding period, we positionally cloned a host factor Fv2 that is required for susceptibility to Friend disease. We determined that Fv2 encodes a truncated version of the receptor tyrosine kinase Stk (Sf-Stk). Besides clear implications for the mechanism of Friend disease, this result is beginning to yield insights into the regulation of erythroid differentiation. Erythroblasts infected with the anemia-inducing strain of Friend virus (FVA cells) undergo Erythropoietin (Epo)-dependent terminal differentiation. FVA cells deprived of Epo undergo apoptosis. Here, we present evidence that Erythropoietin receptor (Epor) signaling not only has a role in FVA cell survival, but is also required for the induction of terminal differentiation. Further, the transcription factors Signal Transducer and Activator of Transcription-5a and -5b (Stat5ab) are required. Our objective is to elucidate the pathway, downstream of Stat5, which induces differentiation. In FVA cells, we hypothesize that Stat5 activates the expression of target genes, which feedback on Sf-Stk and/or Epor signaling to induce terminal differentiation. To test this hypothesis, we propose a series of experiments. Our first aim is to define the Stat5-dependent pathway. We propose to identify direct Stat5 targets with microarrays;to identify Stat5-dependent changes in the signaling pathways downstream of Sf-Stk and Epor;to identify Stat5-dependent changes in cellular proteins and posttranslational modifications;to identify Stat5-dependent changes in protein interactions;and to map sites of post-translational modifications. Our second aim is to examine the role of specific Stat5 targets in FVA cell differentiation. We propose to study the role of known Stat5 targets, Cis and Socs3, in FVA cells derived from Cis-/- mice, and FVA cells derived from Socs3-/- fetal liver cell transplant recipients. We also propose to study the role of a potential Stat5 target, Insulin receptor substrate-2 (Irs2), in FVA cells derived from Irs2-/- mice. Irs2 activates some of the same signaling pathways as the distal Epor. So, we will determine whether Irs2 is functionally redundant with the distal Epor in normal erythroid cells.